Syphilis is a disease caused by Treponema pallidum (hereinafter also referred to as “Tp”) infection. The diagnosis of syphilis is generally made by an immunoassay of anti-Tp antibody in the blood using, for example, the Treponema pallidum Hemagglutination Test (TPHA), the Fluorescent Treponema Antibody Absorption Test (FTA-ABS) and/or the Treponema pallidum Immobilization Test (TPI), as well as enzyme-linked immuno sorbent assay (ELISA) and Western Blot systems. These tests detect antibodies that react with Tp or antigen preparations from Tp, such as the Tp antigens Tp15, Tp17, and Tp47, for example. The TPI test involves a microscopic assessment of the extent to which complement activating antibodies in a patient's serum inhibit the mobility of Tp. This test is not generally used as a diagnostic due to its high cost. The TPHA test involves agglutination of patient serum antibodies with erythrocytes to which the Tp sonicate antigen is bound. The FTA-ABS test involves indirect immunofluorescence microscopy to detect the binding of specific antibodies in patient serum to Tp attached to a solid support via a fluorescent-labelled secondary antibody. Although these assays are widely used, they lack the sensitivity required for detecting early stage or low level infection, when Tp antibody levels in the body fluids are very low.
In addition, the popular use of individual recombinant or purified Tp antigens in existing immunoassays for syphilis does not account for the binding by some antibodies to complexes formed between Tp antigens and other antigens (i.e., Tp antigen binding partners) normally present in the subject's body fluid. Existing immunoassays thus fail to detect such antibodies in the absence of the Tp antigen binding partner, which results in some instances in low sensitivity or false negative assay results. A need therefore exists for pathogen diagnostic assays with improved sensitivity, which detect antibodies that bind to pathogen antigen/binding partner complexes.